Luminescent Bimetallic Lanthanide Bioprobes for Cellular Imaging with Excitation in the Visible-Light Range
Abstract: A series of homoditopic ligands H2LCX (X=4–6) has been designed to self-assemble with lanthanide ions (LnIII), resulting in neutral bimetallic helicates of overall composition [Ln2ACHTUNGTRENUNG(LCX)3] with the aim of testing the influence of substituents on the photophysical properties, particularly the excitation wavelength. The complex species are thermodynamically stable in water (logb23 in the range 26–28 at pH 7.4) and display a metal-ion environment with pseudo-D3 symmetry and devoid of coordinated water molecules. The emission of EuIII, TbIII, and YbIII is sensitised to various extents, depending on the properties of the ligand donor levels. The best helicate is [Eu2ACHTUNGTRENUNG(LC5)3] with excitation maxima at 350 and 365 nm and a quantum yield of 9%. The viability of cervix cancer HeLa cells is unaffected when incubated with up to 500 mm of the chelate during 24 h. The helicate permeates into the cells by endocytosis and locates into lysosomes, which co-localise with the endoplasmatic reticulum, as demonstrated by counterstaining experiments. The relatively long excitation wavelength allows easy recording of bright luminescent images on a confocal microscope (lexc=405 nm). The new lanthanide bioprobe remains undissociated in the cell medium, and is amenable to facile derivatisation. Examination of data for seven EuIII and TbIII bimetallic helicates point to shortcomings in the phenomenological rules of thumb between the energy gap D(3pp*–5DJ) and the sensitisation efficiency of the ligands.